Some facts about nuclear reactor for power generation:
A. Understanding running of nuclear reactor (fission) for power generation –
i. Nuclear power can come from the fission of uranium, plutonium or thorium or the fusion of hydrogen into helium. Today it is almost all uranium. The fission of an atom of uranium produces few million times the energy produced by the combustion of an atom of carbon from coal.
ii. Natural uranium is almost entirely a mixture of two isotopes, U-235 and U-238. Today’s commercial nuclear reactor uses U-235 for fission reaction. Natural uranium has 99.3 percent of U-238 and only 0.7 percent of U-235.
iii. Most nuclear power plants today use enriched uranium in which the concentration of U-235 is increased from 0.7 percent to (nowadays) about 4 to 5 percent.
iv. The U-238 "tails" are left over for eventual use in "breeder reactors". The Canadian CANDU reactors don't require enriched fuel, but since they use expensive heavy water instead of ordinary water, their energy cost is about the same.
v. Present reactors that use only the U-235 in natural uranium are very likely good for some hundreds of years.
vi. A power reactor contains a core with a large number of fuel rods. Each rod is full of pellets of uranium oxide. An atom of U-235 fissions when it absorbs a neutron. The fission produces two fission fragments and other particles that fly off at high velocity. When they stop the kinetic energy is converted to heat.
vii. The steam withdrawn and run through the turbines controls the power level of the reactor. The heat from the fuel rods is absorbed by water which is used to generate steam to drive the turbines that generate the electricity.
viii. After about two years, when enough U-235 is converted to fission products and the fission products have built up enough so that the fuel rods must be removed and replaced by new ones.
ix. Besides fission products, spent fuel rods contain some plutonium produced by the U-238 in the reactor absorbing a neutron. This plutonium and leftover uranium can be separated in a reprocessing plant and used as reactor fuel.
x. Thus running a reactor for four years produces enough plutonium (about 1/4 as much as the U-235 that was in the fuel rods) to run it for one more year provided the plutonium is extracted and put into new fuel rods. Newer designs with higher "burnup ratios" get more of their energy from plutonium.
B. Understanding nuclear waste –
i. After the fuel has been in the reactor for about 18 months, much of the uranium has already fissioned.
ii. A considerable quantity of fission products also have built up in the fuel.
iii. The reactor is then refueled by replacing about 1/3 of the fuel rods. This generally takes one or two months. Canadian CANDU reactors replace fuel continuously.
iv. When fuel rods are removed from the reactor they contain large quantities of highly radioactive fission products and are generating heat at a high rate.
v. They are then put in a large tank of water about the size of a swimming pool. There they become less radioactive as the more highly radioactive isotopes decay and also generate less and less heat.
vi. The fuel rods should then be chemically reprocessed. Reprocessing removes any leftover uranium and the plutonium that has been formed.
vii. The fission products are then put in a form for long term storage.
viii. A large reactor produces about 1.5 tonnes of fission products per year.
ix. Economic advantages of reprocessing are great. If we do not reprocess, we lose the economic benefit of the plutonium.
x. At the same time, the spent fuel remains radioactive for longer duration and has to be better guarded, because it contains plutonium.
C. Understanding of future nuclear reactor - ‘breeder reactor’ -
i. If the design of reactor is such that, enough U-238 can also be converted to plutonium so that after a fuel cycle there is more fissionable material than there was in the original fuel rods in the reactor. This system is more economical.
ii. Such a design is called a ‘breeder reactor’.
iii. Breeder reactors essentially use U-238 as fuel. Therefore, it is more advantageous and it is estimated that, there is 140 times more beneficial than the conventional reactor.
iv. They are more expensive than present reactors.
v. Breeder reactor will be reprocessing on site, so no plutonium will ever become externally available.
vi. It is much safer system than the present one. It is hoped that it wiould address the proliferation concerns of the anti-nukes.
